mu4e: add mu4e-mu-debug for verbosely logging the server

With mu4e-mu-debug set to t, the mu server will be more verbose in its
logging (mu.log or the journal)

mu4e/mu4e-proc.el

@@ -249,10 +249,11 @@ Start the process if needed."
   (unless (and mu4e-mu-binary (file-executable-p mu4e-mu-binary))
     (mu4e-error
      (format
-      "`mu4e-mu-binary' (%S) not found; please set to the path to the mu executable"
+      "`mu4e-mu-binary' (%S) not found; please set to the mu executable path"
       mu4e-mu-binary)))
   (let* ((process-connection-type nil) ;; use a pipe
          (args (when mu4e-mu-home `(,(format"--muhome=%s" mu4e-mu-home))))
+         (args (if mu4e-mu-debug (cons "--debug" args) args))
          (args (cons "server" args)))
     (setq mu4e~proc-buf "")
     (setq mu4e~proc-process (apply 'start-process

mu4e/mu4e-vars.el

@@ -51,6 +51,12 @@ path."
   :group 'mu4e
   :safe 'stringp)
 
+(defcustom mu4e-mu-debug nil
+  "Whether to run the mu binary in debug-mode.
+Setting this to t increases the amount of information in the log."
+  :type 'boolean
+  :group 'mu4e)
+
 (make-obsolete-variable 'mu4e-maildir
                         "determined by server; see `mu4e-root-maildir'." "1.3.8")